While plate armor is the main defense, that dont mean it as the only 1. A gambeson or similar were always underneath, for a few reason such as:
To shock absorb, and to prevent the metal from grinding against the skin/body. Gambeson depending on its amount of layers are more then a valid form of armor.
Yes been wounded in any reality with any armor is still a wound, like how there was special arrows for chainmail, that would slide right in, and break off once you tried to pull it out, and the arrow heads were toxic(think it was bronze). But depending on the era, the armor is either easier or harder to put on or off.
Over the ages plate got heavier and heavier and sure did make them slower, and was indeed later replaced with Cuirass, that could protect well against bullets. They were also lighter as you said, and was not a full plate. Back to plate, back when plate armor was still used, they didt prevent you from standing up once fallen(old myth)... The movies did plate armor dirt with making it out to be so heavy you could not move well at all. Medieval knights were quite mobile on foot, with fine agility compared to todays modern soldiers(depending on the age/era).
Full plate was not a death trap, given how OP they were befor hand cannons, or accurate ones.
@LadyWYT
Plate armor been that much slower, is bit of a movie/video game troupe, same goes for 2 handed weapons been slow.
I know its for balance reasons, but the real life comparison as to why its like this ingame, is not a good argument.
And removing too many of the down sides, would make all other armors obsolete.
TL;DR historically accurate or not, is not a good argument for why anything is how it is, when it dont work in reverse(spears been limited).